Fast and accurate mueller matrix infrared spectroscopic ellipsometer

ABSTRACT

An ellipsometer, polarimeter and the like system operating in the infrared spectral range (0.75 μm to 1000 μm), utilizing a tunable quantum cascade laser (QCL) source in combination with dithering capability to reduce speckle and standing wave effects, dual-rotating optical elements, a single-point detector, as well as optional means of reducing the size of the probe beam at the measurement surface and optional chopper for lock-in detection.

This application Claims Benefit of Provisional Application Ser. No.63/288,441 filed Dec. 10, 2021.

TECHNICAL FIELD

The present invention relates to ellipsometer and polarimeter systems,and more particularly to an ellipsometer, polarimeter or the like systemoperating in the infrared spectral range (0.75 μm to 1000 μm), utilizinga tunable quantum cascade laser source operated in quasi-cw mode incombination with wavelength dithering to reduce the effects of speckleand standing waves, dual-rotating optical elements, and a single-pointdetector. The invention further involves methods of use in real-timemonitoring and mapping of large samples.

BACKGROUND

Ellipsometry is a standard optical characterization technique to measurethe thickness and optical properties of thin films by monitoring therelative polarization change of polarized light reflected from ortransmitted through a sample at oblique angles of incidence andcomparing experimental data with model predicted values. The relevantinformation about the optical properties of the sample constituents andtheir thickness is typically extracted from a regression analysis duringwhich the model parameters are modified with the goal of simultaneouslyminimizing the square error between the experimental data and modelcalculated data for all measured wavelengths and angles of incidence. Ingeneral, an ellipsometer system which is applied to investigate a samplesequentially comprises:

-   -   a) a Source of a beam of electromagnetic radiation,    -   b) a Polarizer element,    -   c) optionally a retarding element,    -   d) additional elements, such as lenses,    -   e) a sample,    -   f) additional elements, such as lenses,    -   g) Optionally a retarding element,    -   h) An Analyzer (polarizer) element, and    -   i) a detector system.

Spectroscopic ellipsometry utilizes many different wavelengths of light,either sequentially or simultaneously, to extract relevant sampleinformation. The technique is commonly applied in the NIR-Vis-UVspectral range where a combination of different light sources such asQTH, Xe, and D2 lamps, and suitable detectors such as Si or InGaAs CCDs(parallel detection), or Si/InGaAs photodiodes or photomultiplier tubes(sequential detection), are combined to cover a wide spectral range.Wavelength selectivity is achieved by either utilizing white light andspreading the different components out over array detectors afterinteraction with the sample (with prisms or gratings), or selecting anarrow spectral portion by means of a monochromator before interactionwith the sample and detection on a single element detector. In thelatter case, the spectral bandwidth must be narrower than the width of aspectral feature of interest.

Modern ellipsometers modulate one or more optical components and extractthe relevant experimental data from a Fourier transformation of themodulated raw detector signal. Instruments for the NIR-Vis-UV spectralrange are commercially available and can measure a full spectrum withinseconds or minutes depending on if all wavelengths are detectedsimultaneously or scanned sequentially. The topic is well described inseveral publications, two such publications are the review papers byCollins et al., titled “Automatic Rotating element ellipsometers:Calibration, Operation and Real-Time Applications” Rev. Sci. Instrum.,61 (8) (1990) [1], and “Dual rotating-compensator multi-channelellipsometer: Instrument development for high-speed Mueller matrixspectroscopy of surfaces and thin films”, Rev. Sci. Instrum., 72 (3)(2001) [2].

Mueller matrix ellipsometers determine not only the standardellipsometric parameters Ψ and Δ defined as the ratio {tilde over (ρ)}of the complex Fresnel reflection/transmission coefficients for p- ands-polarized light (X=r or t):

${\overset{\sim}{\rho} = {\frac{{\overset{\sim}{X}}_{p}}{{\overset{˜}{X}}_{s}} = {\tan{\Psi \cdot e^{i\Delta}}}}},$

which are valid only for samples without cross-polarization such asoptically isotropic samples. Instead, the partial or full Mueller matrixis reported which can account for the effects of cross-polarization fromanisotropic or structured surfaces. The Mueller matrix describes thechange of the polarization state of an incoming light beam afterinteraction with any optical element or sample, where the polarizationstate is given by the Stokes vector S, as defined by

$S = {\begin{bmatrix}S_{0} \\S_{1} \\S_{2} \\S_{3}\end{bmatrix} = \begin{bmatrix}{I_{x} + I_{y}} \\{I_{x} - I_{y}} \\{I_{{+ 45}{^\circ}} - I_{{- 45}{^\circ}}} \\{I_{R} - I_{L}}\end{bmatrix}}$

Here, I_(x), I_(y), I₊₄₅, I⁻⁴⁵, I_(R), and I_(L) are the measuredintensities for horizontal, vertical, +45, −45°, right-circular, andleft-circular polarization, respectively.

The polarization state of the reflected or transmitted light beam isthen given by:

$S_{out} = {\begin{pmatrix}S_{0} \\S_{1} \\S_{2} \\S_{3}\end{pmatrix}_{out} = {{\begin{bmatrix}M_{11} & M_{12} & M_{13} & M_{14} \\M_{21} & M_{22} & M_{23} & M_{24} \\M_{31} & M_{32} & M_{33} & M_{34} \\M_{41} & M_{42} & M_{43} & M_{44}\end{bmatrix}.\begin{pmatrix}S_{0} \\S_{1} \\S_{2} \\S_{3}\end{pmatrix}_{in}} = {{M \cdot S_{in}}\prime}}}$

where M is the unnormalized Mueller matrix. In order to extract, thefull Mueller matrix, the ellipsometer needs to be able to create rightand left circular (or elliptical) polarized light on the input side anddetect right and left circular (or elliptical) polarized light on thedetector side which requires the presence of retarding optical elementson both side of the sample. Absence of one or both retarding elementsprevents measurement of the last row (no retarder on the detector side)or last column (no retarder on the input side).

The continuous rotation of one or more optical elements creates amodulated detector signal where the number of harmonics depends on thesystem type and number of rotating optical elements. A basic rotatinganalyzer ellipsometer without retarding elements has only one frequencycomponent with a sin and cos term (two independent quantities), a singlerotating compensator ellipsometer has two frequency components (4independent quantities), while a dual rotating compensator ellipsometerhas 12 frequency components (24 independent quantities). Thus, for oneconfiguration of fixed optical elements in the beam, only thedual-rotation setup contains enough information in the raw detectorsignal for a full optical cycle to determine the 15 independent elementsof a full normalized Mueller matrix, (i.e. the dual rotating elementsetup can determine either the isotropic ellipsometric parameters or theMueller matrix from the same raw detector signal depending on samplerequirements (an optical cycle for a rotating element ellipsometer isusually the time required until all moving optical elements are back tothe same original position as at the beginning of the cycle)). In fact,there is redundant information in the raw data which might improve theaccuracy of the ellipsometer. The single-rotating element ellipsometerscan create more independent information by repositioning some of thefixed optical elements but require accordingly more optical cycles andthe time to reposition the elements which increases the minimum possiblemeasurement time. Ellipsometer systems with two rotating polarizers areunable to determine the isotropic phase parameter Δ accurately forvalues around 0° or 180°, (i.e. when measuring isotropic transparentsamples). For existing single-rotating Analyzer systems, a fixedcompensator is added to the beam after the Polarizer to add apredefined, fixed amount of retardation, (i.e. producing ellipticalpolarization, which significantly improves the accuracy of thedetermined a values). Similar principle might be applicable to adual-rotating polarizer system as well but was not reported so far.

A continuously-rotating-element ellipsometer exposes the sample to manydifferent polarization states, detects many different polarizationstates, or both, thereby not relying on sensitivity of the measurementto only a few predefined, potentially unfavorable input and outputpolarization state combinations a specific sample is exposed to. TheFourier analysis of the modulated detector signal adds another level ofnoise filtering. The sample information is contained in the relativeamplitude of the different Fourier components, not in absolute intensityinformation. Further, the change of the expected frequency components orappearance of undesirable frequency components due to non-idealities ofthe optical or electronic setup allows identification and correction ofthese non-idealities by calibration and allows identification of driftaway from calibration-determined system parameters.

In many ellipsometer systems, a chopper is added to the system tofacilitate lock-in detection. This chopper can either be a physicaldevice such as a rotating slotted blade added between the source and thefirst polarizing element or electronic means of turning on and off thelight source at defined frequency. For IR detectors without DCsensitivity like many of the common IR detectors, the chopper can beessential to access this information by shifting it to an arbitrarilyselectable carrier frequency. For modulated signals as present inrotating element ellipsometers, the frequency content is obtained asside bands of the carrier frequency which is commonly much higher thanthe modulation frequencies components (without chopper). Use of lock-indetection enables additional noise filter options. Further, opticaldetectors typically have higher sensitivity at increased frequency dueto reduction in 1/f noise. Any system with a chopper added can only beoperated in continuous or quasi-continuous wave mode.

A step-scan system typically identifies the minimum number of opticalelement orientation combinations necessary to theoreticallyoverdetermine the equation system that needs to be solved to extract thedesired ellipsometric quantities, (i.e. the standard ellipsometricparameters or Mueller matrix elements). Typically, ratios of intensityvalues are acquired to account for normalization requirements of the rawdetector signal. The step-scan approach assumes that the system isstable and repeatable over time with regards to optical elementpositioning, temperature, light source intensity and so on.

Snapshot ellipsometers either divide the measurement beam laterally intosub-beams (“division of wave front”) or utilize beam splitters(“division of amplitude”). Both setups rely on stability andrepeatability of the optical beam path (including sample alignment), andtheir sensitivity depends on the optical arrangement which usuallycannot be changed. Calibration of non-idealities in the system andevaluation of calibration drifts over time is much harder due to theabsence of a modulated detector signal.

Commercial ellipsometers for the infrared (IR) spectral range utilizethe FTIR principle where the information for all wavelengths isextracted simultaneously from an interferogram created by aninterferometer light source, (e.g. facilitating a globar source incombination with a Michelson interferometer, and detected with asingle-point detector which prevents continuous modulation of theoptical elements and slows down the measurement process significantly).Instead, a full optical cycle for each wavelength is createdsequentially by stepping the “rotating” optical element through equallyspaced orientations and obtaining a full spectrum for each orientationbefore moving on to the next one. The minimum required number ofdiscrete steps depends on the number of harmonics created with thespecific ellipsometer type, (e.g. 4 harmonics for a single rotatingcompensator ellipsometer require at least 9 discrete compensatororientations to resolve these 4 frequency components according toNyquist theorem). Dual-rotating compensator systems to measure the fullMueller matrix accordingly would require at least 49 orientations whichwould not be feasible in practice. Spectrometers based on the FTIRprinciple report spectra of fixed wavelength spacing where theresolution depends on the maximum path difference in the two arms of theinterferometer. Narrow spectral features, even if only present in alimited part of the spectrum, require finer resolution measurementswhich increases measurement time inversely proportional to improvementin resolution. Further, the thermal light sources utilized in theinterferometer source emit low light intensity and have large divergenceresulting in long averaging times and preventing focusing of the lightbeam to a small measurement spot, thereby limiting sample throughput,preventing mapping or real-time measurements for process monitoring, andlimiting the minimum size of samples that can be studied. Typicalmeasurements of advanced data types such as Generalized ellipsometry orMueller matrix which often also require the measurement at multiplesample azimuths can take several days per sample even on asingle-rotating compensator ellipsometer.

Continuing, recently tunable, monochromatic quantum cascade lasers (QCL)for the IR spectral range became commercially available. Thesehigh-intensity, coherent light sources are widely tunable with spectralbandwidth <1 cm⁻¹ and enable the application of similar designprinciples as used recently for the NIR-Vis-UV spectral includingcontinuous rotation of optical elements, focusing, and fast sequentialdetection of a multitude of relevant wavelengths. Only these narrowbandwidth, tunable sources allow measurements at a pre-selectable set ofwavelengths which can be uniquely tailored to the characteristicabsorption features of a sample of interest. QCL's are typicallyoperated in a pulsed mode with repetition frequencies in the MHz range.Optical systems utilizing these QCL sources can either detect individualpulses assuming stable pulse-to-pulse intensity and fast enoughdetectors are used, or they can be operated in quasi-continuous wavemode where many pulses are averaged per detected data point. Thedivergence of the QCL beam is typically better than 5 mrad which allowsfocusing of the collimated beam to diffraction limited spot sizes ofless than 200 μm by means of IR transparent lenses, for example made oflow-dispersion ZnSe, or by reflective focusing optics made of non-planarmirrors. The monochromatic nature of the QCL further allows use ofadjustable focusing optics, which optimize the position of the optic fora given frequency in order to account for focus shifts as a consequenceof chromatic aberration in the lens system. This allows diffractionlimited focusing of the beam for a wide range of frequencies. Althoughnot related to Patentability of the present invention, it is note thatone relevant patent in this area is U.S. Pat. No. 8,351,036 to Liphardt.

A unique characteristic of a QCL source, often explored in otherapplications but detrimental here, is the very long coherence of theemitted light compared to conventional thermal light sources. For thepurpose of ellipsometry measurements, a long coherence length isunfavorable for several reasons. Coherent reflections from the backsideof a thick substrate cause interference oscillations in the data, (i.e.oscillations in Ψ and Δ versus frequency (i.e. energy units) caused byconstructive and destructive interference of light reflected from thesample surface and backside)). The relatively large thickness of typicalsubstrates relative to the wavelength of the probing light leads tospectrally very narrow spacing of the interference pattern and slightestsubstrate thickness non-uniformity causes an envelope of theinterference maxima and minima that is very hard to model as exactoptical constants and thickness values are required to match thecharacteristic pattern. On the other hand, totally incoherent lightreflections from the backside and front of the substrate hitting thedetector only lead to slight shifts in Ψ and Δ, and algorithms exist toeasily account for this type of incoherent backside reflection in thedata. Existing patents using QCL sources deal with this problem, forexample, by implementing a knife edges to suppress the backsidereflections as disclosed in U.S. Pat. No. 10,901,241. Another verysignificant aspect of the long coherence length is the formation ofspeckle and standing wave patterns which strongly and randomly influencethe data versus frequency. Speckle is the result of scattering on roughsurfaces like a sample or the surface of an optical element in the beampath caused by very small path length differences and interference ofdifferent parts of the initially collimated beam on the detector. Thespeckle pattern is very sensitive to variations in the optical beam pathsuch as structural modification as a result of pressure applied to aphoto-elastic modulator, mechanical rotation of an optical element, ormechanical shifts of optical element positions as a result of anangle-of-incidence change, for example. Standing wave patterns arisefrom certain reflections within the optical system which can occurbetween any interfaces that the beam crosses at normal incidence, in theentire beam path from the source to the detector, and with potentialdistances between the reflecting surfaces only limited by the coherencelength of the light source. The formation of Standing Waves is, inprinciple, similar to the creation of speckle, but while for specklemany sub-beams of the same wave front interfere to cause a spatialvariation of intensity on the detector, for standing waves, differentwave trains reflected and transmitted at different parallel interfacescause an interference patterns with respect to different wavelength. Infact, the presence of such standing waves can only be seen whenmeasuring many narrowly spaced frequencies. It is very hard to identifythe source of these patterns as path differences can be a singlewavelength, for example within a wire grid polarizer or it could be manymultiples of the wavelength, for example an interference between thedetector window and the first polarizer. These patterns show up in thedata at unpredictable spectral positions and are impossible to accountfor by calibration. Even if identified, elimination of the sources isoften impossible. One potential way of elimination disclosed here is afast, longitudinal repositioning of one the causing optical interfacesaround its center position, (i.e. movement along the beam path, but themechanical movement would need to be faster than the averaging time ofthe detector). A primary concern in most ellipsometry applicationspatents is maximization of speed of operation. Effective reduction ofthe coherence length for individually measured pulses of the QCL is noteasily possible and the respective systems must suffer from accuracyissues in one form or another either due to speckle or standing waves.This is a non-trivial problem and effective mitigation strategies areneeded to overcome these issues. Here, we emphasize that the presentinvention, instead of dealing with the coherence, seeks to circumventthe resulting problems by shortening the coherence using electronic,mechanical, or optical measures, averaging of many pulses by use ofrelatively slow detectors or pulse train integration on fasterdetectors, and by using large area single point detectors that collectmost of the speckle pattern. The disclosed systems are thereforeentirely different in nature from previously disclosed QCL-basedellipsometer systems as well as traditional spectroscopic ellipsometersystems, which do not use a coherent light source.

Reduction of the coherence length of a highly coherent light source suchas a QCL is a non-trivial problem. Different methods for monochromaticlight were developed for example to mitigate speckle effect inlaser-based projectors. Common mechanical and/or optical approachesinclude the use of rough spinning discs, vibrating transparentmembranes, the use of long multimode fibers (sometimes in combinationwith stretching or vibration), polarization scrambling in anisotropicoptics and perhaps others. All these approaches can be applied incombination with QCL light sources in the ellipsometer designs proposedhere. However, in a preferred embodiment, a different approach, based onwavelength scrambling is preferred. That is, the approach if averagingmany pulses of slightly different wavelengths around a centralmeasurement wavelength. In commercially available QCL systems, thewavelength can be set very quickly using external trigger modes whichessentially apply small steps to the motors that control the position ofthe grating in the external grating cavity that selects the lasingwavelengths. By “dithering” the grating around the set wavelengthposition, for example periodically or randomly adding and subtractingsmall angle shifts from the set position, slightly different wavelengthscan be set in very quick succession. Using pulse averaging on thedetector, the perceived bandwidth of the laser line is effectivelybroadened. Since coherence length and broadening of a laser line areinversely proportional to each other, this is equivalent to a reductionof the coherence length. Consequently, this approach is only applicablewhen many pulses are averaged on the detector, in stark contrast toexisting patents. The wavelength dithering approach can be applied incombination with common alternative speckle reduction schemes.

The light emitted from a QCL is linearly polarized. In certainsituations it might be desirable to rotate the polarization state tooptimize light throughput through the ellipsometer, for exampleutilizing an “odd-bounce” system as described in a patent to Herzingerat al. (U.S. Pat. No. 6,795,184).

The accuracy of the ellipsometer system for a wide range of samplesrelies heavily on the achievable retardation of the retarding opticalelements and extinction ratio of the polarizing elements. Retarderdesigns based on birefringence of the retarder material are not suitablefor the IR spectral range since no natural materials with large enoughbirefringence are known. However, common designs based on total internalreflection are applicable. Custom shapes of prisms are commerciallyavailable, for example made of ZnSe which is transparent between 0.45and 21 μm. Wire grid polarizers, either free-standing or on a suitable,transparent substrate, are commercially available for the infraredspectral range.

While generally allowing for faster data acquisition, the use ofPhoto-Elastic Modulators (PEM's) as retarding devices in a standard orMueller matrix ellipsometer is generally considered less desirable dueto temperature sensitivity, calibration drift over time, memory effects,and other non-idealities. Further, the underlying math to describe theentire system, specifically in the presence of non-idealities, is muchmore complicated than for a rotating-optical element system. Existingpatents specify an adjustable retardance for each wavelength to keep theretardance value at optimized values of 90° which simplify the mathconsiderably. Note, that retardance values for compensators, based ontotal internal reflections are designed to optimal values fordual-rotation systems which are different from 900, vary slightly withwavelength due to dispersion, and are fixed after assembly of the optic.The signal-to-noise ratio of the measured data in any ellipsometersystem is generally believed to depend on the total number of photonscollected per optical cycle and averaging of several hundred modulationcycles is typically required for PEM systems to achieve satisfactory SNRlevels negating some if not all of the speed advantage over a systemwith mechanically modulated optical elements and much higher photoncount per single optical cycle. A PEM-based ellipsometer with only onePEM in the beam path only reports two normalized frequency componentswhich is insufficient to determine even the full sample information ofan isotropic sample (Standard ellipsometry parameters V and A over thefull range from 0-360°). More data content can be created bymechanically moving other optical elements in the beam path andremeasuring the same sample or adding more PEM units in the beam path.To measure a full Mueller matrix, a total of four PEM must be includedin the system, all operated at finely synchronized unique frequencies.The resulting math requires expansion in terms or Bessel functions andbecomes even more complicated in the presence of non-idealities.Therefore, all PEM-based QCL systems reported in the literature or aspatent only are equipped with a single PEM unit which is not sufficientto measure the full Mueller matrix. All these systems are mainlyapplicable for isotropic samples without cross-polarization whichsignificantly limits their usefulness for many modern applications insemiconductor metrology, metamaterials, and other current fields ofinterest. The present invention could, but does not utilize PEM's.

A Search for relevant prior art has provided the following patents andpeer-reviewed publications:

-   -   U.S. Pat. No. 5,042,951 discloses a laser based ellipsometer        operated with a HeNe laser at 633 nm.    -   Furchner et al. reported on mapping of organic films using a        HeNe laser based single wavelength IR laser ellipsometer        operated in the rotating-analyzer mode in a publication titled        “Fast IR Laser Mapping Ellipsometry for the Study of Functional        Organic Thin Films”, Analyst, 140, 1791-1797(2015).    -   Patent DE 10 2016 202 971 A1 discloses a snapshot ellipsometer        for the infrared spectral range having a high-brilliance        monochromatic light source operated in a single-shot mode where        the probing beam is split into multiple sub-beams after        interaction with the sample and detected by multiple        polarization state detectors or different polarization filtering        properties. Fast single-point detectors are required to resolve        individual shots. The ellipsometric parameters are extracted by        simultaneous consideration of the multiple detector signals for        each shot of the light source.    -   In two related publications by the same authors titled        “Hyperspectral Infrared Laser Polarimetry for Single-Shot        Phase-Amplitude Imaging of Thin Films”, Opt. Lett., Vol. 44, No        19 (2019), and “Sub-Second Infrared Broadband-Laser Single Shot        Phase-Amplitude Polarimetry of Thin Films”, Opt. Lett., Vol. 44,        No. 17 (2019), Furchner et al. demonstrated time-resolved        measurements and mapping of biologic samples in a standard        ellipsometry mode using the snapshot ellipsometer.    -   The publication “Ultrasensitive Broadband Infrared 4×4        Mueller-Matrix Ellipsometry for studies of depolarizing and        anisotropic thin films” published by Furchner et al. in J. Vac.        Sci. Technol. B 38, 014003 (2020) reveals a broadband 4×4        Mueller matrix ellipsometer with retractable achromatic        retarders and various sets of tandem polarizers utilizing an        FTIR source with a globar. Measurements are performed using a        step-scan principle with only a few selected polarizer and        compensator positions measured and combined to extract the        Mueller matrix of the sample.    -   US patents and Publications, U.S. Pat. No. 10,775,149 B1, US        2020/0363332 A1. U.S. Pat. No. 10,901,241 and 11,162,897        disclose a small measurement spot metrology device for at least        partially transparent multilayer structure characterization        based on an ellipsometer with tunable QCL source and at least        one photo-elastic modulator as retarding device, adjusted to        produce the same amount of retardation for each wavelength. The        patents further outline means for failure mode detection of the        QCL, backside reflection suppression utilizing a knife edge, and        spatial filtering to reduce the effective numerical aperture        seen by the detector.    -   Ebner et al. reported a QCL based ellipsometer in “Sub-Second        Quantum Cascade Laser Based Infrared Spectroscopic        Ellipsometry”, Opt. Lett., Vol. 44, No 15 (2019). Their system        uses a single-phase modulator on the input side to measure the        standard ellipsometric parameters Ψ and Δ.    -   Patent US 2020/0240907 A1 discloses a metrology device based on        the FTIR principle and, though practically not reasonable, in        one embodiment illumination is provided by one or more QCL        sources.    -   A paper by Lee et al. titled “Dual Rotating-Compensator        Multichannel Ellipsometer: Instrument Development for High-Speed        Mueller Matrix Spectroscopy of Surfaces and Thin Films”        published in Rev. Sci. Instrum., Vol. 72 No. 3 (2001) outlines        the principle and calibration routines of a dual-rotating        compensator ellipsometer in the NIR-Vis-UV spectral range with        parallel CCD detection.    -   U.S. Pat. Nos. 8,736,838 and 8,705,032 disclose in their        preferred embodiment, a THz ellipsometer system which employs        the dual-rotation of optical elements in combination with a        frequency-tunable coherent backwards-wave oscillator source and        a Golay cell detector.    -   While not critical to Patentability, it is noted that U.S. Pat.        No. 6,084,675 describes an adjustable beam alignment        compensator/retarder with application to spectroscopic        ellipsometry. Also, U.S. Pat. No. 6,141,102 describes a single        triangular shaped optical retarder element. U.S. Pat. No.        5,946,098 describes a dual tipped wire grid polarizer in        combination with multiple retarders. U.S. Pat. No. 6,084,674        describes a dual horizontally oriented triangular shaped optical        retarder. U.S. Pat. No. 6,084,674 describes a parallelogram        shaped optical retarder element. U.S. Pat. Nos. 7,450,231 and        7,460,230 describe deviation angle self-compensating retarder        systems.

The present invention deviates from the prior art in that it combineshighly tunable, brilliant QCL laser sources operated in quasi-cw modewith a coherence length reduction scheme, the dual-mechanical rotationof optical elements in the beam path, and use of averaging single-pointdetectors. The invention also utilizes the dual-continuous rotation withsequential scanning of the wavelength in order to extract a modulateddetector signal that can be used to accurately calibrate theellipsometer and extract the most accurate data from a sample. Coherencelength scrambling circumvents critical issues that limit data accuracysuch as standing wave patterns and speckle. The system can determineadvanced data types such as Generalized ellipsometry and Mueller matrixfor anisotropic or scattering samples. Mueller matrix or Generalizedellipsometry data can be determined from the same single optical cycledata as the standard ellipsometric parameters Ψ and Δ which presents asignificant speed advantage over a single-rotating element system. Aprototype has been built to demonstrate these unique capabilities notreported so far in any other patent or reported in the literature. Someexemplary data is included in the accompanying Detailed Description andDrawings section.

While the forging disclosure shows that it is known to apply QuantumCascade Lasers as Source of Electromagnetic Radiation in Ellipsometersand the like, it is emphasized that it has not been known to applySpeckle and Standing Wave Reducing Means thereto. A Search at the USPTOWebsite for patent including (Quantum and (Cascade and (Laser and(Speckle and (Ellipsometer))))) turned up no hits. A similar Search wasconducted for Published applications and returned only one hit. That isUS2005/0249667. Said Published application, however, does not discloseanything remotely like the invention presented herein. A patent whichdiscusses QCL's and dithering, U.S. Pat. No. 10,267,903 is mentioned asit is known. Further. USPTO Data Base Searching for patents by knownmanufacturers (patent Assignees), for Quantum Cascade Lasers, (e.g.Daylight and Block Engineering) fails to show that use thereof inellipsometers and polarimeters was considered. While Daylight doesconsider Speckle in some of its patents, that is not in conjunction withuse in ellipsometry or polarimetry.

Even in view of the prior art, need remains for ellipsometers,polarimeters and the like systems which operate in the infrared spectralrange (0.75 μm to 1000 μm), which ellipsometers and polarimeters utilizea tunable quantum cascade laser source, wherein speckle and standingwaves are reduced via coherence length control achieved by dithering, incombination with dual-rotating optical elements, a single-pointdetector, and that can include optional means of reducing the size ofthe probe beam at the measurement surface and use of a chopper tosynchronize the detector signal.

DISCLOSURE OF THE INVENTION

In broad terms, the present invention is a mid-infrared ellipsometer orpolarimeter system with a high-brilliance, low-divergence, tunable lightsource in combination with a single-point detector utilizing thedual-optical element rotation principle and operated in quasi-cw mode,including Speckle and Standing Wave effects mitigation via coherencewavelength dithering.

The present invention is, in its preferred embodiment, a method ofinvestigating a sample which reduces the effects of speckle and standingwaves, comprising the steps of:

a) providing an ellipsometer or polarimeter system comprising:

-   -   a′) a quantum cascade laser source of high-brilliance, tunable        electromagnetic radiation with emission wavelengths in the        mid-infrared spectral range, said quantum cascade laser source        comprising means for dithering the electromagnetic radiation        output;    -   b′) a beam polarizing optical element;    -   c′) a first rotatable optical retarder or polarizer element;    -   d′) a stage for supporting a sample;    -   e′) a second rotatable optical retarder or polarizer element;    -   f′) a beam analyzing polarizer optical element;    -   g′) a single-point detector for infrared radiation which is not        capable of resolving individual pulses of quantum cascade laser        electromagnetic radiation.        Said ellipsometer or polarimeter system is characterized in        that:    -   during use said quantum cascade laser source of electromagnetic        radiation operates in a continuous or quasi-continuous wave        mode, (i.e. pulses are not detected individually); and    -   during use the wavelength content of said continuous or        quasi-continuous wave is dithered so that cyclically or randomly        a sequential plurality of different wavelengths around a central        wavelength is output; and    -   during use said detector provides output data based on an        average of a multiplicity of pulses in said continuous or        quasi-continuous dithered electromagnetic radiation input        thereto; and    -   during use said rotatable optical retarder or polarizing        elements continuously rotate at different frequencies of fixed        ratio.

Said Method continues with:

-   -   b) placing a sample on said stage for supporting a sample;    -   c) while causing said rotatable optical retarder or polarizing        elements continuously rotate at different frequencies of fixed        ratio, causing said quantum cascade laser source to provide a        dithered continuous or quasi-continuous beam of electromagnetic        radiation directed so that it passes through said:        -   beam polarizing optical element; and        -   first rotatable optical retarder or polarizer element;        -   interacts with said sample on said stage for supporting a            sample;            then passes through said:    -   second rotatable optical retarder or polarizer element; and    -   beam analyzing polarizer optical element; and    -   enters said single-point detector for infrared radiation;    -   d) causing said single-point detector for infrared radiation to        provide sample characterizing output data based upon said        received dithered continuous or quasi-continuous beam of        electromagnetic radiation; and    -   e) analyzing the data output by said single-point detector to        provide sample characterizing information.

The step of providing ellipsometer or polarimeter system can involveproviding a system characterized by at least one selection from thegroup consisting of:

-   -   a″) said detector is characterized by a selection from the group        consisting of:        -   DTGS;        -   MCT;        -   LiTaO3;        -   PbS;        -   PbSe;        -   InSb;        -   a QWIP detector; and        -   a Si bolometer;    -   b″) said beam polarizing and analyzing optical elements are        free-standing or substrate-bonded wire grid polarizers;    -   c″) said first and second retarding or polarizer optical        elements are dual Fresnel rhomb retarders, a single-triangle        retarder, a dual-triangular shaped retarder, or a        parallelogram-shaped retarder;    -   d″) said beam polarizing and analyzing optical elements are        movable along the path of the beam of electromagnetic radiation;    -   e″) said first rotatable retarder or polarizer optical element        is a rotatable polarizer optical element;    -   f″) said second rotatable retarder or polarizer optical element        is a rotatable polarizing optical element;    -   g″) said first rotatable retarder or polarizer optical element        is a rotatable retarder optical element;    -   h″) said second rotatable optical retarder or polarizer element        is a rotatable retarder optical element;    -   i″) a polarization state rotator is placed between the source        and the beam polarizing element;    -   j″) an additional movable polarizer is provided in front of the        beam polarizing element; and    -   k″) where lenses are added between the rotatable optical        retarder or polarizing elements and the sample to reduce the        size or the measurement spot on the sample surface.

Said method can involve, in step e), performing a regression based ondata obtained in step e) onto a mathematical model of the sample.

Said method can further involve changing the relative distance, alongthe path of the beam of electromagnetic radiation, between at least twosystem elements selected from the group consisting of:

-   -   a′) said quantum cascade laser source of high-brilliance,        tunable electromagnetic radiation with emission wavelengths in        the mid-infrared spectral range, said quantum cascade laser        source comprising means for dithering the electromagnetic        radiation output;    -   b′) said beam polarizing optical element;    -   c′) said first rotatable optical retarder or polarizer element;    -   d′) said stage for supporting a sample;    -   e′) a second rotatable optical retarder or polarizer element;    -   f′) said beam analyzing polarizer optical element; and g′) said        single-point detector for infrared radiation which is not        capable of resolving individual pulses of quantum cascade laser        electromagnetic radiation;        and obtaining data sets for each resulting configuration, then        performing a simultaneous regression of all said data sets onto        a mathematical model of said sample.

This simultaneous regression approach can be especially beneficiallyapplied when the changed distance involved is between:

-   -   f′) said beam analyzing polarizer optical element; and    -   g′) said single-point detector for infrared radiation which is        not capable of resolving individual wavelengths of quantum        cascade laser electromagnetic radiation;        and Standing Waves present a problem that is nearly impossible        to account for in a mathematical model. The simultaneous        regression onto multiple data sets can serve to average out, or        even cancel the effects of Standing Waves between said f′) and        g′) elements. If the position of said system elements can be        changed much faster than the averaging time of the detector,        averaging out of the effect might occur in a single data set        without the need for simultaneous regression.

At this point, to aid with understanding the benefit of the simultaneousregression approach using multiple data sets, it should be appreciatedthat coherence length in a beam or electromagnetic radiation is thesource of speckle and standing wave problems in sample investigation.That is, both speckle and standing waves have their source in coherencelength. Interference at the detector occurs if all of the back and forthreflected beams in the entire system can interfere. This is based on thesame principle as the speckle effects, but for the speckle, a lot ofparallel parts of the beam that interfere. For the standing waves,different wave trains reflected between parallel interfaces interfere.The difficulty as regards standing waves is that since the lengthdifference is long, it is very hard to identify which interfaces causethe effect. The path difference can be a single wavelength, e.g. withina wire grid polarizer or it could be many multiples of the wavelength,e.g. an interference between the detector window and the firstpolarizer. Shortening of the coherence length avoids these standingwaves as well as speckle.

Continuing, the present invention can also be described as anellipsometer or polarimeter system sequentially comprising:

a) a high-brilliance, tunable quantum cascade laser source orelectromagnetic radiation with emission wavelengths in the mid-infraredspectral range;

b) a beam polarizing optical element;

c) a first rotatable optical retarder or polarizer element;

d) a stage for supporting a sample;

e) a second rotatable optical retarder or polarizer element;

f) a beam analyzing polarizing optical element;

g) a single-point detector of infrared radiation.

Said ellipsometer is characterized by:

-   -   said rotatable optical retarder or polarizer elements        continuously rotate at different frequencies of fixed ratio;    -   said tunable quantum cascade laser source of electromagnetic        radiation operates in a continuous or quasi-continuous wave mode        and does not resolve individual laser pulses, but rather        averages a multiplicity thereof when obtaining data; and    -   said ellipsometer system further comprises a grating which is        periodically or randomly changed in position to add and subtract        small angle shifts from a nominal wavelength set position,        thereby providing slightly different wavelengths at different        times which reduces coherence length and thereby reduce speckle.    -   (It is noted that the grating is typically present in a quantum        cascade laser system per se., which is an integral part of the        ellipsometer or polarimeter system).

An alternative recitation of a method of investigating a samplecomprises the steps or:

a) providing an ellipsometer or polarimeter system comprising:

-   -   a′) a high-brilliance, tunable quantum cascade laser source of        electromagnetic radiation with emission wavelengths in the        mid-infrared spectral range including dithering capability;    -   b′) a beam polarizing optical element;    -   c′) a first rotatable optical retarder or polarizer element;    -   d′) a stage for supporting a sample;    -   e′) a second rotatable optical retarder or polarizer element;    -   f′) a beam analyzing polarizer optical element;    -   g′) a single-point detector of infrared radiation;        said ellipsometer or polarimeter being characterized by:    -   said rotatable optical retarder or polarizing elements        continuously rotate at different frequencies of fixed ratio; and    -   said quantum cascade laser source of electromagnetic radiation        operates in a continuous or quasi-continuous wave mode and does        not resolve individual laser pulses, but rather averages a        multiplicity thereof when obtaining data;

b) applying said ellipsometer system to investigate a sample system withcharacteristic, sharp, spectral features in narrow ranges of the IRspectral range by customizing the measurement resolution spectrally tobe able to fully resolve the features while limiting the number of datapoints in “flat” spectral areas.

Another recitation or a present invention method for investigating asample comprises the steps of:

a) providing an ellipsometer system comprising:

-   -   a′) a high-brilliance, tunable quantum cascade laser source of        electromagnetic radiation with emission wavelengths in the        mid-infrared spectral range;    -   b′) a beam polarizing optical element;    -   c′) a first rotatable optical retarder or polarizer element;    -   d′) a stage for supporting a sample;    -   e′) a second rotatable optical retarder or polarizer element;    -   f′) a beam analyzing polarizer optical element;    -   g′) a single-point detector for infrared radiation.        Said ellipsometer or polarimeter is characterized by:    -   said rotatable optical retarder or polarizing elements        continuously rotate at different frequencies of fixed ratio;    -   said source of electromagnetic radiation operates in a        continuous or quasi-continuous wave mode and does not resolve        individual laser pulses, but rather averages a multiplicity        thereof when obtaining data;    -   said ellipsometer system further comprises a grating which is        periodically or randomly changed in position to add and subtract        small angle shifts from a nominal wavelength set position,        thereby providing slightly different wavelengths at different        times which reduces coherence length and thereby reduce speckle        and standing waves;    -   said stage for supporting a sample being within a chamber with        IR transparent windows that allow the electromagnetic radiation        to pass through the windows at normal incidence.

Said method continues with:

b) placing a sample on said sample stage and causing a beam ofelectromagnetic radiation produced by said high-brilliance, tunablesource of a beam of electromagnetic radiation with emission wavelengthsin the mid-infrared spectral range to pass through said IR transparentwindows and interact with the sample mounted on said stage forsupporting a sample;

c) setting the wavelength output emitted from the source ofelectromagnetic radiation to one wavelength, or a discrete, small,pre-defined subset of wavelengths suitable to characterize modificationof said sample within said chamber and repeatedly cycling through saidsubset of wavelengths to monitor the change of sample properties versustime.

Another present invention method of investigating a sample comprises thesteps of:

-   -   a) providing an ellipsometer or polarimeter system comprising:        -   a′) a high-brilliance, tunable quantum cascade laser source            of electromagnetic radiation with emission wavelengths in            the mid-infrared spectral range;        -   b′) a beam polarizing optical element;        -   c′) a first rotatable optical retarder or polarizer element;        -   d′) a stage for supporting a sample;        -   e′) a second rotatable optical retarder or polarizer            element;        -   f′) a beam analyzing polarizer optical element;        -   g′) a single-point detector of infrared radiation;            said ellipsometer or polarimeter being characterized by:    -   said rotatable optical retarder or polarizing elements        continuously rotate at different frequencies of fixed ratio;    -   said quantum cascade laser source of electromagnetic radiation        operates in a continuous or quasi-continuous wave mode and does        not resolve individual laser pulses, but rather averages a        multiplicity thereof when obtaining data; and    -   said ellipsometer system further comprises a grating which is        periodically or randomly changed in position to add and subtract        small angle shifts from a nominal wavelength set position,        thereby providing slightly different wavelengths at different        times which reduces coherence length and thereby reduce        speckle; b) placing a sample on said sample stage and causing a        beam or electromagnetic radiation produced by said        high-brilliance, tunable source of a beam of electromagnetic        radiation with emission wavelengths in the mid-infrared spectral        range to interact with the sample mounted on said stage for        supporting a sample and enter said single point detector;    -   c) setting the wavelength output emitted from the source of        electromagnetic radiation to one wavelength, or a discrete,        small, pre-defined subset of wavelengths suitable to        characterize the variation of the sample properties, and while        moving the sample under the measurement beam observing data        provided by said single point detector output.

It is noted that as in the case of the first recited Method herein, inany of the foregoing ellipsometer or polarimeter systems or methods ofuse, the ellipsometer can be characterized by at least one selectionfrom the group consisting of:

-   -   DTGS;    -   MCT;    -   LiTaO3;    -   PbS;    -   PbSe;    -   InSb;    -   a QWIP detector; and    -   a Si bolometer;    -   b″) said beam polarizing and analyzing optical elements are        free-standing or substrate-bonded wire grid polarizers;    -   c″) said first and second retarding or polarizer optical        elements are dual Fresnel rhomb retarders, a single-triangle        retarder, a dual-triangular shaped retarder, or a        parallelogram-shaped retarder;    -   d″) said beam polarizing and analyzing optical elements are        movable along the path of the beam of electromagnetic radiation;    -   e″) said first rotatable retarder or polarizer optical element        is a rotatable polarizer optical element;    -   f″) said second rotatable retarder or polarizer optical element        is a rotatable polarizing optical element;    -   g″) said first rotatable retarder or polarizer optical element        is a rotatable retarder optical element;    -   h″) said second rotatable optical retarder or polarizer element        is a rotatable retarder optical element;    -   i″) a polarization state rotator is placed between the source        and the beam polarizing element;    -   j″) an additional movable polarizer is provided in front of the        beam polarizing element; and    -   k″) where lenses are added between the rotatable optical        retarder or polarizing elements and the sample to reduce the        size or the measurement spot on the sample surface.

Further, in any of the foregoing providing an ellipsometer system canfurther comprise at least one selection from the group consisting of:

-   -   a chopper between source and beam polarizing optical element and        electronic means to synchronize the detector signal to the        chopper frequency lock-in detection;    -   a stationary retarder after the first rotatable element to        present elliptically polarized electromagnetic radiation        thereafter;    -   a speckle reducer between said quantum cascade laser source of        infrared electromagnetic radiation and said single point        detector thereof;    -   a fixed compensator between said quantum cascade laser source of        infrared electromagnetic radiation and said single point        detector thereof;    -   focusing and re-collimation elements before and after said stage        for and supporting a sample respectively;    -   focusing and re-collimation lenses which are mounted to allow        movement along the path of said beam of electromagnetic        radiation before and after said stage for and supporting a        sample respectively; and    -   selecting said first and second rotatable optical retarder or        polarizer elements from the group consisting of:        -   the first is a retarder optical element and the second is a            polarizer optical element;        -   the second is a retarder optical element and the first is a            polarizer optical element;        -   both first and second optical elements are polarizer            elements;        -   both first and second optical elements are retarder            elements.

The present invention will be better understood by reference to theDetailed Description Section of this Specification in conjunction withthe Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a general embodiment of an ellipsometer with tworetarding elements and optional focusing lenses.

FIG. 2A shows a dual-rotating element (both compensators) embodiment ofan ellipsometer with a QCL light source

FIG. 2B shows a dual-rotating element (polarizer and compensator)embodiment of an ellipsometer with a QCL light source.

FIG. 2C shows a dual-rotating element (polarizer and analyzer)embodiment of an ellipsometer with a QCL light source

FIG. 2D shows a dual-rotating element (compensator and analyzer)embodiment of an ellipsometer with a QCL light source.

FIG. 3A shows a dual-rotating element (both compensators) embodiment ofan ellipsometer with a QCL light source, including a speckle reducer(SR).

FIG. 3B shows a dual-rotating element (polarizer and compensator)embodiment or an ellipsometer with a QCL light source, including aspeckle reducer (SR).

FIG. 3C shows a dual-rotating element (polarizer and analyzer)embodiment of an ellipsometer with a QCL light source, including aspeckle reducer (SR).

FIG. 3D shows a dual-rotating element (compensator and analyzer)embodiment of an ellipsometer with a QCL light source, including aspeckle reducer (SR).

FIG. 4A shows a dual-rotating element (both compensators) embodiment ofan ellipsometer with a QCL light source, including a polarization staterotating element (PSR).

FIG. 4B shows a dual-rotating element (polarizer and compensator)embodiment of an ellipsometer with a QCL light source, including apolarization state rotating element (PSR)

FIG. 4C shows a dual-rotating element (polarizer and analyzer)embodiment of an ellipsometer with a QCL light source, including apolarization state rotating element (PSR).

FIG. 4D shows a dual-rotating element (compensator and analyzer)embodiment of an ellipsometer with a QCL light source, including apolarization state rotating element (PSR).

FIG. 5A shows a dual-rotating element ellipsometer attached to theoutside of a chamber with sample mounted inside the chamber.

FIG. 5B shows a dual-rotating element ellipsometer including a specklereducer (SR), attached to the outside of a chamber with sample mountedinside the chamber.

FIG. 6 shows a measurement profile where the resolution of the QCLfrequency steps is adjusted spectrally to resolve an absorption featureof interest.

FIG. 7 shows experimental and best-match model generated data for a 120nm thermal oxide film on silicon substrate.

FIG. 8 depicts experimental and best-match model generated data for aglass slide measured on a dual-rotating element ellipsometer.

FIG. 9 shows experimental and best-match model generated data for asilicon wafer with native oxide at a fixed frequency of 1400 cm-1 versusangle of incidence demonstrating the accuracy of the prototype vs. angleof incidence.

DETAILED DESCRIPTION

FIG. 1 depicts an exemplary embodiment of an ellipsometer with tworetarding elements and optional focusing lenses. The ellipsometersequentially comprises a light source (LS), an optional chopper (Cn), aPolarizer (P) (i.e. a polarizing optical element), a compensator on theinput side (C) (i.e. a retarding optical element), an optionalinput-side lens assembly (L), a measured sample (MS) mounted on a stage(STG), an equivalent lens assembly on the detector side (L), adetector-side compensator (C′) (i.e. a retarding optical element), anAnalyzer (A) (polarizing optical element), and a single-pointspectroscopic detector (D).

FIGS. 2A-2D show dual-rotating element ellipsometer designs with a QCLlight source with wavelength dithering and different elements rotating.Also shown is a monitor (MON) which displays a representative plot ofthe effective QCL output profile after applying wavelength dithering.FIG. 2A depicts a modification in which the Polarizer (P) and Analyzer(A) are fixed and the input-side and detector-side compensators (C,C′)are continuously rotated in sync but at different frequencies to createa modulated raw signal on the single point detector. In FIG. 2B, theinput-side compensator is replaced by another Polarizer (P′) which iscontinuously rotating in sync but at different frequency than thedetector-side compensator (C′). In FIG. 2C, the detector-sidecompensator is replaced by a second Analyzer (A′) which is continuouslyrotating in sync with the input-side compensator but at differentfrequency. In FIG. 2D, the detector-side compensator is replaced by asecond Analyzer (A′) which is continuously rotating in sync with theinput-side Polarizer but at different frequency. A fixed Compensator (C)can optionally be added to the beam path to add a predefined, fixedamount or retardation to the beam resulting in elliptically polarizedlight interacting with the sample, for example to improve the accuracywhen measuring transparent samples.

FIGS. 3A-3D depict equivalent setups as in FIGS. 2A-2D, but instead ofwavelengths dithering, the coherence length is reduced using a specklereducer (SR). Also shown is a plotted indication of the effect of thespeckle reducer on a spatially resolved detector signal, (i.e. a speckleintensity (I) pattern without speckle reducer and smooth intensitypattern with speckle reducer added to the beam).

FIGS. 4A-4D depict equivalent setups as in FIGS. 2A-2D, but anadditional polarization state rotating element (PSR) is inserted in thebeam path between the QCL and the first polarizing element (P). As inFIGS. 2A-2D, a monitor (MON) is shown which provides insight to the QCLoutput when applying the wavelength dithering.

FIGS. 5A and 5B depict a dual-rotating element ellipsometer with QCLlight source mounted on a chamber so that the beam of electromagneticradiation emitted from the source assembly can pass through an IRtransparent window (W), interact with a sample mounted on a stage withinthe chamber, pass through another IR transparent window (W′), and bedetected by the detector assembly. In FIG. 5A, the system useswavelength dithering while in FIG. 5B, a speckle reducer (SR) reducesthe coherence length of the QCL. FIG. 5 b shows the effect of thespeckle reducer (SR) on a spatially resolved detector signal.

FIG. 6 shows a customized measurement profile where the resolution ofthe QCL frequency steps is adjusted spectrally to resolve an absorptionfeature of interest. Note, the more narrow absorption feature at lowerfrequencies is measured with a finer frequency resolution than the widerfeature at high frequencies. Between the absorption features, a singleor a few data points are sufficient to determine, for example, thethickness of a thin film in the spectral range where the film istransparent and light can travel through the film and be reflected atinterfaces with subjacent layers or the substrate. The spectral profileof a typical QCL is overlayed to show that even fine resolution of lessthan 10 cm⁻¹ steps are feasible with adequate bandwidth for eachfrequency point.

FIG. 7 shows experimental and best-match model generated data for a 120nm thermal oxide film on silicon substrate measured on a dual-rotatingelement ellipsometer prototype with tunable QCL source. Excellent matchbetween model and experiment is achieved by using standard materialproperties from a library and only matching the thickness of the thinfilm which demonstrates the accuracy of the instrument versus manyfrequencies.

FIG. 8 depicts experimental and best-match model generated data for aglass slide measured on a dual-rotating element ellipsometer prototypewith tunable QCL source demonstrating the accuracy of the ellipsometerfor measurements on transparent substrates.

FIG. 9 shows experimental and best-match model generated data for asilicon wafer with native oxide at a fixed frequency of 1400 cm-1 versusangle of incidence demonstrating the accuracy of the prototype vs. angleof incidence.

While the Specification is written sufficiently broad to include othersources of electromagnetic radiation, it is to be understood that thepresent invention is found in the use of a tunable quantum cascade lasersource in combination with the presence or Speckle Reducing element(s)and/or practices.

It is also noted that that the word “Dither” can be replaced with thewords “Wavelength Scrambling”, “Wavelength Shifting”, or similarvariations.

Further, it should be appreciated that FIGS. 1-5B indicate that elementsof the system:

-   -   a′) said quantum cascade laser source of high-brilliance,        tunable electromagnetic radiation with emission wavelengths in        the mid-infrared spectral range, said quantum cascade laser        source comprising means for dithering the electromagnetic        radiation output;    -   b′) said beam polarizing optical element;    -   c′) said first rotatable optical retarder or polarizer element;    -   d′) said stage for supporting a sample;    -   e′) a second rotatable optical retarder or polarizer element;    -   f′) said beam analyzing polarizer optical element; and    -   g′) said single-point detector for infrared radiation which is        not capable of resolving individual pulses of quantum cascade        laser electromagnetic radiation;        are physically sequentially separated from one another. Present        invention methodology discloses that separation distances        between elements can be significant as regards data obtained        when wavelengths in the IR range are involved. The present        invention provides for adjustment of separation distances during        data collection, including the collection of different data sets        corresponding to different element separations, and practice of        simultaneous regression of a mathematical model thereonto.

Having hereby disclosed the subject matter of the present invention, itshould be obvious that many modifications, substitutions, and variationsof the present invention are possible in view of the teachings. It istherefore to be understood that the invention may be practiced otherthan as specifically described and should be limited in breadth andscope only by the Claims.

We claim:
 1. A method of investigating a sample with electromagneticradiation in the mid-infrared spectral range, which reduces the effectsof speckle and standing waves, comprising the steps of: a) providing anellipsometer or polarimeter system comprising: a′) a quantum cascadelaser source of high-brilliance, tunable electromagnetic radiation withemission wavelengths in the mid-infrared spectral range, said quantumcascade laser source comprising means for dithering the electromagneticradiation output; b′) a beam polarizing optical element; c′) a firstrotatable optical retarder or polarizer element; d′) a stage forsupporting a sample; e′) a second rotatable optical retarder orpolarizer element; f′) a beam analyzing polarizer optical element; g′) asingle-point detector for infrared radiation which is not capable ofresolving individual pulses of quantum cascade laser electromagneticradiation; said method being characterized in that: during use saidquantum cascade laser source of electromagnetic radiation operates in acontinuous or quasi-continuous wave, so that individual pulses are notdetected; and during use the wavelength content of said continuous orquasi-continuous wave is dithered so that cyclically or randomly asequential plurality of different wavelengths around a centralwavelength is output; and during use said detector provides output databased on an average of a multiplicity of pulses in said continuous orquasi-continuous dithered electromagnetic radiation input thereto; andduring use said rotatable optical retarder or polarizing elementscontinuously rotate at different frequencies of fixed ratio; said methodfurther comprising: b) placing a sample on said stage for supporting asample; c) while causing said rotatable optical retarder or polarizingelements continuously rotate at different frequencies of fixed ratio,causing said quantum cascade laser source to provide a ditheredcontinuous or quasi-continuous beam of electromagnetic radiationdirected so that it passes through said: beam polarizing opticalelement; and first rotatable optical retarder or polarizer element;interacts with said sample on said stage for supporting a sample; thenpasses through said: second rotatable optical retarder or polarizerelement; and beam analyzing polarizer optical element; and enters saidsingle-point detector for infrared radiation; d) causing saidsingle-point detector for infrared radiation to provide samplecharacterizing output data based upon said received dithered continuousor quasi-continuous beam of electromagnetic radiation; and e) analyzingthe data output by said single-point detector to provide samplecharacterizing information.
 2. A method as in claim 1, in which the stepof providing ellipsometer or polarimeter system involves providing asystem characterized by at least one selection from the group consistingof: a″) said detector is characterized by a selection from the groupconsisting of: DTGS; MCT; LiTaO3; PbS; PbSe; InSb; a QWIP detector; anda Si bolometer; b″) said beam polarizing and analyzing optical elementsare free-standing or substrate-bonded wire grid polarizers; c″) saidfirst and second retarding or polarizer optical elements are dualFresnel rhomb retarders, a single-triangle retarder, a dual-triangularshaped retarder, or a parallelogram-shaped retarder; d″) said beampolarizing and analyzing optical elements are movable along the path ofthe beam of electromagnetic radiation; e″) said first rotatable retarderor polarizer optical element is a rotatable polarizer optical element;f″) said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 3. A method as in claim 1 inwhich step e) involves performing a mathematical regression based ondata obtained in step d) onto a mathematical model of the sample.
 4. Amethod as in claim 1, which further comprises changing the relativedistance, along the path of the beam of electromagnetic radiation,between at least two system elements selected from the group consistingof: a′) said quantum cascade laser source of high-brilliance, tunableelectromagnetic radiation with emission wavelengths in the mid-infraredspectral range, said quantum cascade laser source comprising means fordithering the electromagnetic radiation output; b′) said beam polarizingoptical element; c′) said first rotatable optical retarder or polarizerelement; d′) said stage for supporting a sample; e′) a second rotatableoptical retarder or polarizer element; f′) said beam analyzing polarizeroptical element; and g′) said single-point detector for infraredradiation which is not capable of resolving individual pulses of quantumcascade laser electromagnetic radiation; and, in step d) obtaining datasets for each resulting configuration, then in step e) performing asimultaneous mathematical regression of all said data sets onto amathematical model of said sample.
 5. A method as in claim 4, in whichthe simultaneous mathematical regression is applied to data setsobtained when the changed distance involved is between: f′) said beamanalyzing polarizer optical element; and g′) said single-point detectorfor infrared radiation which is not capable of resolving individualwavelengths of quantum cascade laser electromagnetic radiation.
 6. Amethod as in claim 1, which further comprises changing the relativedistance, along the path of the beam of electromagnetic radiation,between at least two system elements selected from the group consistingof: a′) said quantum cascade laser source of high-brilliance, tunableelectromagnetic radiation with emission wavelengths in the mid-infraredspectral range, said quantum cascade laser source comprising means fordithering the electromagnetic radiation output; b′) said beam polarizingoptical element; c′) said first rotatable optical retarder or polarizerelement; d′) said stage for supporting a sample; e′) a second rotatableoptical retarder or polarizer element; f′) said beam analyzing polarizeroptical element; and g′) said single-point detector for infraredradiation which is not capable of resolving individual pulses of quantumcascade laser electromagnetic radiation; and in step d) changing atleast one distance much fast than the averaging time of the detector fora single raw data point, thus averaging out the standing wave.
 7. Amethod as in claim 6, in which the changed distance involved is between:f′) said beam analyzing polarizer optical element; and g′) saidsingle-point detector for infrared radiation which is not capable ofresolving individual wavelengths of quantum cascade laserelectromagnetic radiation.
 8. An ellipsometer or polarimeter systemcomprising: a′) a quantum cascade laser source of high-brilliance,tunable electromagnetic radiation with emission wavelengths in themid-infrared spectral range, said quantum cascade laser sourcecomprising means for dithering the electromagnetic radiation output; b′)a beam polarizing optical element; c′) a first rotatable opticalretarder or polarizer element; d′) a stage for supporting a sample; e′)a second rotatable optical retarder or polarizer element; f′) a beamanalyzing polarizer optical element; g′) a single-point detector forinfrared radiation which is not capable of resolving individual pulsesof quantum cascade laser electromagnetic radiation; said ellipsometer orpolarimeter system being characterized in that: during use said quantumcascade laser source of electromagnetic radiation operates in acontinuous or quasi-continuous wave mode and does not resolve individuallaser pulses; and during use the wavelength content of said continuousor quasi-continuous wave is dithered so that cyclically or randomly asequential plurality of different wavelengths around a centralwavelength is output; and during use said detector provides output databased on an average of a multiplicity of pulses in said continuous orquasi-continuous dithered electromagnetic radiation input thereto; andduring use said rotatable optical retarder or polarizing elementscontinuously rotate at different frequencies of fixed ratio; such thatin use a sample is placed on said stage for supporting a sample, andwhile causing said rotatable optical retarder or polarizing elementscontinuously rotate at different frequencies of fixed ratio, saidquantum cascade laser source is caused to provide a dithered continuousor quasi-continuous beam of electromagnetic radiation directed so thatit passes through said: beam polarizing optical element; and firstrotatable optical retarder or polarizer element; interacts with saidsample on said stage for supporting a sample; then passes through said:second rotatable optical retarder or polarizer element; and beamanalyzing polarizer optical element; and enters said single-pointdetector for infrared radiation; such that said single-point detector ofinfrared radiation provides sample characterizing output data based uponsaid received dithered continuous or quasi-continuous beam ofelectromagnetic radiation, and said data output by said single-pointdetector provides sample characterizing information.
 9. An ellipsometeror polarimeter system as in claim 8, which involves providing a systemcharacterized by at least one selection from the group consisting of:a″) said detector is characterized by a selection from the groupconsisting or: DTGS; MCT; LiTaO3; PbS; PbSe; InSb; a QWIP detector; anda Si bolometer; b″) said beam polarizing and analyzing optical elementsare free-standing or substrate-bonded wire grid polarizers; c″) saidfirst and second retarding or polarizer optical elements are dualFresnel rhomb retarders, a single-triangle retarder, a dual-triangularshaped retarder, or a parallelogram-shaped retarder; d″) said beampolarizing and analyzing optical elements are movable along the path ofthe beam of electromagnetic radiation; e″) said first rotatable retarderor polarizer optical element is a rotatable polarizer optical element;f″) said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 10. An ellipsometer orpolarimeter system sequentially comprising: a) a high-brilliance,tunable quantum cascade laser source of electromagnetic radiation withemission wavelengths in the mid-infrared spectral range; b) a beampolarizing optical element; c) a first rotatable optical retarder orpolarizer element; d) a stage for supporting a sample; e) a secondrotatable optical retarder or polarizer element; f) a beam analyzingpolarizing optical element; g) a single-point detector of infraredradiation; said ellipsometer being characterized by: said rotatableoptical retarder or polarizer elements continuously rotate at differentfrequencies of fixed ratio; said tunable quantum cascade laser source ofelectromagnetic radiation operates in a continuous or quasi-continuouswave mode and does not resolve individual laser pulses, but ratheraverages a multiplicity thereof when obtaining data; and saidellipsometer system further comprises a movable grating which isperiodically or randomly changed in position to add and subtract smallangle shifts from a nominal wavelength set position, thereby providingslightly different wavelengths at different times which reducescoherence length and thereby reduce speckle and standing waves.
 11. Anellipsometer or polarimeter system as in claim 10, is furthercharacterized by at least one selection from the group consisting of:a″) said detector is characterized by a selection from the groupconsisting of: DTGS; MCT; LiTaO3; PbS; PbSe; InSb; a QWIP detector; anda Si bolometer; b″) said beam polarizing and analyzing optical elementsare free-standing or substrate-bonded wire grid polarizers; c″) saidfirst and second retarding or polarizer optical elements are dualFresnel rhomb retarders, a single-triangle retarder, a dual-triangularshaped retarder, or a parallelogram-shaped retarder; d″) said beampolarizing and analyzing optical elements are movable along the path orthe beam or electromagnetic radiation; e″) said first rotatable retarderor polarizer optical element is a rotatable polarizer optical element;f″) said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 12. A method of investigating asample comprising the steps of: a) providing an ellipsometer orpolarimeter system comprising: a′) a high-brilliance, tunable quantumcascade laser source of electromagnetic radiation with emissionwavelengths in the mid-infrared spectral range including ditheringcapability; b′) a beam polarizing optical element; c′) a first rotatableoptical retarder or polarizer element; d′) a stage for supporting asample; e′) a second rotatable optical retarder or polarizer element;f′) a beam analyzing polarizer optical element; g′) a single-pointdetector of infrared radiation; said ellipsometer being characterizedby: said rotatable optical retarder or polarizing elements continuouslyrotate at different frequencies of fixed ratio; and said quantum cascadelaser source of electromagnetic radiation operates in a continuous orquasi-continuous wave mode and does not resolve individual laser pulses,but rather averages a multiplicity thereof when obtaining data; b)applying said ellipsometer system to investigate a sample system withcharacteristic, sharp, spectral features in narrow ranges of the IRspectral range by customizing the measurement resolution spectrally tobe able to fully resolve the features while limiting the number of datapoints in “flat” spectral ranges.
 13. A method as in claim 12, in whichthe step of providing an ellipsometer or polarimeter is furthercharacterized by said ellipsometer or polarimeter comprising at leastone selection from the group consisting of: a″) said detector ischaracterized by a selection from the group consisting of: DTGS; MCT;LiTaO3; PbS; PbSe; InSb; a QWIP detector; and a Si bolometer; b″) saidbeam polarizing and analyzing optical elements are free-standing orsubstrate-bonded wire grid polarizers; C″) said first and secondretarding or polarizer optical elements are dual Fresnel rhombretarders, a single-triangle retarder, a dual-triangular shapedretarder, or a parallelogram-shaped retarder; d″) said beam polarizingand analyzing optical elements are movable along the path of the beam ofelectromagnetic radiation; e″) said first rotatable retarder orpolarizer optical element is a rotatable polarizer optical element; f″)said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 14. A method of investigating asample comprising the steps of: a) providing an ellipsometer systemcomprising: a′) a high-brilliance, tunable quantum cascade laser sourceof electromagnetic radiation with emission wavelengths in themid-infrared spectral range; b′) a beam polarizing optical element; c′)a first rotatable optical retarder or polarizer element; d′) a stage forsupporting a sample which is present within a chamber comprising IRtransparent windows; e′) a second rotatable optical retarder orpolarizer element; f′) a beam analyzing polarizer optical element; g′) asingle-point detector for infrared radiation; said ellipsometer beingcharacterized by: said rotatable optical retarder or polarizing elementscontinuously rotate at different frequencies of fixed ratio; and saidsource of electromagnetic radiation operates in a continuous orquasi-continuous wave mode and does not resolve individual laser pulses,but rather averages a multiplicity thereof when obtaining data; and saidellipsometer system further comprises a movable grating which isperiodically or randomly changed in position to add and subtract smallangle shifts from a nominal wavelength set position, thereby providingslightly different wavelengths at different times which reducescoherence length and thereby reduce speckle and thus speckle; such thatduring use the wavelength content of said continuous or quasi-continuouswave is dithered so that cyclically or randomly a sequential pluralityof different wavelengths around a central wavelength (“set wavelength”)is output; and during use said detector provides output data based on anaverage of a multiplicity of pulses of slightly different wavelengths insaid continuous or quasi-continuous dithered electromagnetic radiationinput thereto; b) placing a sample on said sample stage and causing abeam of electromagnetic radiation produced by said high-brilliance,tunable source of a beam of electromagnetic radiation with emissionwavelengths in the mid-infrared spectral range to pass through said IRtransparent windows and interact with the sample mounted on said stagefor supporting a sample; c) setting the wavelength output emitted fromthe source of electromagnetic radiation to one wavelength, orsequentially to a discrete, small, pre-defined subset of wavelengthssuitable to characterize modification of said sample within said chamberand repeatedly cycling through said subset of wavelengths to monitor thechange of sample properties versus time.
 15. A method as in claim 14, inwhich the step of providing an ellipsometer or polarimeter is furthercharacterized by said ellipsometer or polarimeter comprising at leastone selection from the group consisting of: a″) said detector ischaracterized by a selection from the group consisting of: DTGS; MCT;LiTaO3; PbS; PbSe; InSb; a QWIP detector; and a Si bolometer; b″) saidbeam polarizing and analyzing optical elements are free-standing orsubstrate-bonded wire grid polarizers; C″) said first and secondretarding or polarizer optical elements are dual Fresnel rhombretarders, a single-triangle retarder, a dual-triangular shapedretarder, or a parallelogram-shaped retarder; d″) said beam polarizingand analyzing optical elements are movable along the path of the beam ofelectromagnetic radiation; e″) said first rotatable retarder orpolarizer optical element is a rotatable polarizer optical element; f″)said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 16. A method of investigating asample comprising the steps of: a) providing an ellipsometer systemcomprising: a′) a high-brilliance, tunable quantum cascade laser sourceof electromagnetic radiation with emission wavelengths in themid-infrared spectral range; b′) a beam polarizing optical element; c′)a first rotatable optical retarder or polarizer element; d′) a movablestage for supporting a sample; e′) a second rotatable optical retarderor polarizer element; f′) a beam analyzing polarizer optical element;g′) a single-point detector of infrared radiation; said ellipsometerbeing characterized by: said rotatable optical retarder or polarizingelements continuously rotate at different frequencies of fixed ratio;and said quantum cascade laser source of electromagnetic radiationoperates in a continuous or quasi-continuous wave mode and does notresolve individual laser pulses, but rather averages a multiplicitythereof when obtaining data; and said ellipsometer system furthercomprises a grating which is periodically or randomly changed inposition to add and subtract small angle shifts from a nominalwavelength set position, thereby providing slightly differentwavelengths at different times which reduces coherence length andthereby reduce speckle; such that during use the wavelength content ofsaid continuous or quasi-continuous wave is dithered so that cyclicallyor randomly a sequential plurality of different wavelengths around acentral wavelength (“set wavelength”) is output; and during use saiddetector provides output data based on an average of a multiplicity ofpulses of slightly different wavelengths in said continuous orquasi-continuous dithered electromagnetic radiation input thereto; b)placing a sample on said movable sample stage and causing a beam ofelectromagnetic radiation produced by said high-brilliance, tunablesource of a beam of electromagnetic radiation with emission wavelengthsin the mid-infrared spectral range to interact with the sample mountedon said stage for supporting a sample and enter said single pointdetector; c) setting the wavelength output emitted from the source ofelectromagnetic radiation to one wavelength, or sequentially to adiscrete, small, pre-defined subset of wavelengths suitable tocharacterize the variation of the sample properties, and while movingthe sample under the measurement beam observing data provided by saidsingle point detector output.
 17. A method as in claim 16, in which thestep of providing an ellipsometer system is further characterized by atleast one selection from the group consisting of: a″) said detector ischaracterized by a selection from the group consisting of: DTGS; MCT;LiTaO3; PbS; PbSe; InSb; a QWIP detector; and a Si bolometer; b″) saidbeam polarizing and analyzing optical elements are free-standing orsubstrate-bonded wire grid polarizers; c″) said first and secondretarding or polarizer optical elements are dual Fresnel rhombretarders, a single-triangle retarder, a dual-triangular shapedretarder, or a parallelogram-shaped retarder; d″) said beam polarizingand analyzing optical elements are movable along the path of the beam ofelectromagnetic radiation; e″) said first rotatable retarder orpolarizer optical element is a rotatable polarizer optical element; f″)said second rotatable retarder or polarizer optical element is arotatable polarizing optical element; g″) said first rotatable retarderor polarizer optical element is a rotatable retarder optical element;h″) said second rotatable optical retarder or polarizer element is arotatable retarder optical element; i″) a polarization state rotator isplaced between the source and the beam polarizing element; j″) anadditional movable polarizer is provided in front of the beam polarizingelement; and k″) where lenses are added between the rotatable opticalretarder or polarizing elements and the sample to reduce the size of themeasurement spot on the sample surface.
 18. A method as in claim 1, inwhich the step of providing an ellipsometer system further comprises atleast one selection from the group consisting of: providing a chopperbetween source and beam polarizing optical element and electronic meansto synchronize the detector signal to the chopper frequency lock-indetection; providing a stationary retarder after the first rotatableelement to present elliptically polarized electromagnetic radiationthereafter; providing a speckle reducer between said quantum cascadelaser source of infrared electromagnetic radiation and said single pointdetector thereof; providing a fixed compensator between said quantumcascade laser source of infrared electromagnetic radiation and saidsingle point detector thereof; providing focusing and re-collimationelements before and after said stage for and supporting a samplerespectively; providing focusing and re-collimation lenses which aremounted to allow movement along the path of said beam of electromagneticradiation before and after said stage for and supporting a samplerespectively; and selecting said first and second rotatable opticalretarder or polarizer elements from the group consisting of: the firstis a retarder optical element and the second is a polarizer opticalelement; the second is a retarder optical element and the first is apolarizer optical element; both first and second optical elements arepolarizer elements; both first and second optical elements are retarderelements.
 19. A method as in claim 6, in which the step of providing anellipsometer system further comprises at least one selection from thegroup consisting of: providing a chopper between source and beampolarizing optical element and electronic means to synchronize thedetector signal to the chopper frequency lock-in detection; providing astationary retarder after the first rotatable element to presentelliptically polarized electromagnetic radiation thereafter; providing aspeckle reducer between said quantum cascade laser source of infraredelectromagnetic radiation and said single point detector thereof;providing a fixed compensator between said quantum cascade laser sourceof infrared electromagnetic radiation and said single point detectorthereof; providing focusing and re-collimation elements before and aftersaid stage for and supporting a sample respectively; providing focusingand re-collimation lenses which are mounted to allow movement along thepath of said beam of electromagnetic radiation before and after saidstage for and supporting a sample respectively; and selecting said firstand second rotatable optical retarder or polarizer elements from thegroup consisting of: the first is a retarder optical element and thesecond is a polarizer optical element; the second is a retarder opticalelement and the first is a polarizer optical element; both first andsecond optical elements are polarizer elements; both first and secondoptical elements are retarder elements.
 20. A method as in claim 8, inwhich the step of providing an ellipsometer system further comprises atleast one selection from the group consisting of: providing a chopperbetween source and beam polarizing optical element and electronic meansto synchronize the detector signal to the chopper frequency lock-indetection; providing a stationary retarder after the first rotatableelement to present elliptically polarized electromagnetic radiationthereafter; providing a speckle reducer between said quantum cascadelaser source of infrared electromagnetic radiation and said single pointdetector thereof; providing a fixed compensator between said quantumcascade laser source of infrared electromagnetic radiation and saidsingle point detector thereof; providing focusing and re-collimationelements before and after said stage for and supporting a samplerespectively; providing focusing and re-collimation lenses which aremounted to allow movement along the path of said beam of electromagneticradiation before and after said stage for and supporting a samplerespectively; and selecting said first and second rotatable opticalretarder or polarizer elements from the group consisting of: the firstis a retarder optical element and the second is a polarizer opticalelement; the second is a retarder optical element and the first is apolarizer optical element; both first and second optical elements arepolarizer elements; both first and second optical elements are retarderelements.
 21. A method as in claim 10, in which the step of providing anellipsometer system further comprises at least one selection from thegroup consisting of: providing a chopper between source and beampolarizing optical element and electronic means to synchronize thedetector signal to the chopper frequency lock-in detection; providing astationary retarder after the first rotatable element to presentelliptically polarized electromagnetic radiation thereafter; providing aspeckle reducer between said quantum cascade laser source of infraredelectromagnetic radiation and said single point detector thereof;providing a fixed compensator between said quantum cascade laser sourceof infrared electromagnetic radiation and said single point detectorthereof; providing focusing and re-collimation elements before and aftersaid stage for and supporting a sample respectively; providing focusingand re-collimation lenses which are mounted to allow movement along thepath of said beam of electromagnetic radiation before and after saidstage for and supporting a sample respectively; and selecting said firstand second rotatable optical retarder or polarizer elements from thegroup consisting of: the first is a retarder optical element and thesecond is a polarizer optical element; the second is a retarder opticalelement and the first is a polarizer optical element; both first andsecond optical elements are polarizer elements; both first and secondoptical elements are retarder elements.
 22. A method as in claim 12, inwhich the step of providing an ellipsometer system further comprises atleast one selection from the group consisting of: providing a chopperbetween source and beam polarizing optical element and electronic meansto synchronize the detector signal to the chopper frequency lock-indetection; providing a stationary retarder after the first rotatableelement to present elliptically polarized electromagnetic radiationthereafter; providing a speckle reducer between said quantum cascadelaser source of infrared electromagnetic radiation and said single pointdetector thereof; providing a fixed compensator between said quantumcascade laser source of infrared electromagnetic radiation and saidsingle point detector thereof; providing focusing and re-collimationelements before and after said stage for and supporting a samplerespectively; providing focusing and re-collimation lenses which aremounted to allow movement along the path of said beam of electromagneticradiation before and after said stage for and supporting a samplerespectively; and selecting said first and second rotatable opticalretarder or polarizer elements from the group consisting of: the firstis a retarder optical element and the second is a polarizer opticalelement; the second is a retarder optical element and the first is apolarizer optical element; both first and second optical elements arepolarizer elements; both first and second optical elements are retarderelements.
 23. A method as in claim 13, in which the step of providing anellipsometer system further comprises at least one selection from thegroup consisting of: providing a chopper between source and beampolarizing optical element and electronic means to synchronize thedetector signal to the chopper frequency lock-in detection; providing astationary retarder after the first rotatable element to presentelliptically polarized electromagnetic radiation thereafter; providing aspeckle reducer between said quantum cascade laser source of infraredelectromagnetic radiation and said single point detector thereof;providing a fixed compensator between said quantum cascade laser sourceof infrared electromagnetic radiation and said single point detectorthereof; providing focusing and re-collimation elements before and aftersaid stage for and supporting a sample respectively; providing focusingand re-collimation lenses which are mounted to allow movement along thepath of said beam of electromagnetic radiation before and after saidstage for and supporting a sample respectively; and selecting said firstand second rotatable optical retarder or polarizer elements from thegroup consisting of: the first is a retarder optical element and thesecond is a polarizer optical element; the second is a retarder opticalelement and the first is a polarizer optical element; both first andsecond optical elements are polarizer elements; both first and secondoptical elements are retarder elements.
 24. A method as in claim 15, inwhich the step of providing an ellipsometer system further comprises atleast one selection from the group consisting of: providing a chopperbetween source and beam polarizing optical element and electronic meansto synchronize the detector signal to the chopper frequency lock-indetection; providing a stationary retarder after the first rotatableelement to present elliptically polarized electromagnetic radiationthereafter; providing a speckle reducer between said quantum cascadelaser source of infrared electromagnetic radiation and said single pointdetector thereof; providing a fixed compensator between said quantumcascade laser source of infrared electromagnetic radiation and saidsingle point detector thereof; providing focusing and re-collimationelements before and after said stage for and supporting a samplerespectively; providing focusing and re-collimation lenses which aremounted to allow movement along the path of said beam of electromagneticradiation before and after said stage for and supporting a samplerespectively; and selecting said first and second rotatable opticalretarder or polarizer elements from the group consisting of: the firstis a retarder optical element and the second is a polarizer opticalelement; the second is a retarder optical element and the first is apolarizer optical element; both first and second optical elements arepolarizer elements; both first and second optical elements are retarderelements.
 25. A method as in claim 1, in which the step of providing anellipsometer or polarimeter involves said first and second rotatableoptical retarder or polarizer elements are both polarizer elements. 26.A system as in claim 6, in which the ellipsometer or polarimeterinvolves said first and second rotatable optical retarder or polarizerelements are both polarizer elements.
 27. A system as in claim 8, inwhich the ellipsometer or polarimeter involves said first and secondrotatable optical retarder or polarizer elements are both polarizerelements.